This invention relates to the field of optics and lasers, and more specifically to a method and apparatus for pumping optical waveguide lasers formed on a glass substrate.
The telecommunications industry commonly uses optical fibers to transmit large amounts of data in a short time. One common light source for optical-fiber communications systems is a laser formed using erbium-doped glass. One such system uses erbium-doped glass fibers to form a laser that emits at a wavelength of about 1.536 micrometer and is pumped by an infrared source operating at a wavelength of about 0.98 micrometer. One method usable for forming waveguides in a substrate is described in U.S. Pat. No. 5,080,503 issued Jan. 14, 1992 to Najafi et al., which is hereby incorporated by reference. A phosphate glass useful in lasers is described in U.S. Pat. No. 5,334,559 issued Aug. 2, 1994 to Joseph S. Hayden, which is also hereby incorporated by reference. An integrated optic laser is described in U.S. Pat. No. 5,491,708 issued Feb. 13, 1996 to Malone et al., which is also hereby incorporated by reference.
There is a need in the art for an integrated optical system, including one or more high-powered lasers along with routing and other components, that can be inexpensively mass-produced. The system should be highly reproducible, accurate, and stable.
The present invention is embodied by a laser component that includes a glass substrate doped with one or more optically active lanthanide species and having a plurality of waveguides defined by channels within the substrate. (As used herein, a xe2x80x9cchannel within the substratexe2x80x9d is meant to broadly include any channel formed on or in the substrate, whether or not covered by another structure or layer of substrate.) Each substrate waveguide (or xe2x80x9cchannelxe2x80x9d) is defined within the substrate as a region of increased index of refraction relative to the substrate. The glass substrate is doped with one or more optically active lanthanide species which can be optically pumped (typically a rare-earth element such as Er, Yb, Nd, or Pr or a combination of such elements such as Er and Yb) to form a laser medium which is capable of lasing at a plurality of frequencies. Mirrors or distributed Bragg reflection gratings may be located along the length of a waveguide for providing feedback to create a laser-resonator cavity. One or more of the mirrors or reflection gratings is made partially reflective for providing laser output.
The laser component may constitute a monolithic array of individual waveguides in which the waveguides of the array form laser resonator cavities with differing resonance characteristics (e.g., resonating at differing wavelengths). The component may thus be used as part of a laser system outputting laser light at a plurality of selected wavelengths. In certain embodiments of the invention, the resonance characteristics of a waveguide cavity are varied by adjusting the width of the channel formed in the substrate which thereby changes the effective refractive index of the waveguide. The effective refractive index can also be changed by modifying the diffusion conditions under which the waveguides are formed as described below. A diffraction Bragg reflector (DBR) grating formed into or close to the waveguide is used, in some embodiments, to tune the wavelength of light supported in the waveguide cavity. Changing the effective refractive index thus changes the effective wavelength of light in the waveguide cavity which determines the wavelengths of the longitudinal modes supported by the cavity. In another embodiment, the resonance characteristics of the waveguide cavities are individually selected by varying the pitch of the DBR reflection gratings used to define the cavities which, along with the effective refractive index for the propagated optical mode, determines the wavelengths of light reflected by the gratings. In still other embodiments, the location of the gratings on the waveguide is varied in order to select a laser-resonator cavity length that supports the desired wavelength of light.
In some embodiments, the laser element is constructed from a glass substrate which is a phosphate alkali glass doped with a rare-earth element such as Er or Yb/Er. The channels defining the waveguides are created by exposing a surface of the substrate to an ion-exchange solvent through a mask layer having a plurality of line apertures corresponding to the channels which are to be formed. The ion exchange may be carried out through an aluminum mask layer in an aluminum or borosilicate glass crucible using molten potassium nitrate as a solvent. Lessened etching of the substrate by the ion-exchange melt has been found to occur in some embodiments that use a tightly sealed aluminum crucible having a graphite gasket between opposing flanges that are tightly bolted together, and having two reserviors, one for holding the salt melt away from the glass wafers during heating (and later cooling) and another reservior for holding the salt melt in contact with the glass wafers during ion-exchange processing. In other embodiments, a borosilicate crucible is used and if the potassium nitrate is pre-baked at a temperature of at least 120 degrees C for 24-48 hours in an inert argon atmosphere. In other embodiments, the crucible is placed inside a fully enclosed chamber during the ion-exchange process, with the chamber filled with an inert atmosphere. Carrying out the ion-exchange process in an enclosed chamber has been found to lessen surface etching due to oxidation reactions. The exchange of K for Na in the substrate produces a channel in the substrate of higher refractive index than the rest of the substrate, thus defining a waveguide. In another particular embodiment, a sodium nitrate electrode is used to carry out electrical field-assisted diffusion of Na ions into the substrate after the K-diffused waveguides are formed. This has the effect of driving the waveguides deeper into the substrate and giving them a more circular cross section. The buried waveguides thus avoid the effects of corrosive processes that result in surface etching.
In one embodiment, a surface-relief grating forming a distributed Bragg reflection grating is fabricated on the surface of the waveguide by coating the surface with photoresist, defining the grating pattern in the photoresist holographically or through a phase mask, developing the photoresist pattern, and etching the grating pattern into the waveguide with a reactive ion system such as an argon ion mill. In certain embodiments, a more durable etch mask allowing more precise etching and higher bias voltages is obtained by depositing chromium on the developed photoresist pattern using an evaporation method which causes the chromium to deposit on the tops of the grating lines.